I am surprised of the total lack of these cups in the pouches sold by all the vendors out there. I think one reason is that our armed forces are not using the P90 in great numbers. Look at the evolution in military gear since the GWOT started.
Sadly, the market for P90 kit may be driven in part by (for back of a better term) hobbyists, shooting enthusiasts, and airsofters. Hopefully this will change as more military and LEO use the weapon. Manufacturers need to look beyond the current market trend. Most pouches are the same design, with a few different buckles and velcro.
If you think about it, the magazine mouth needs to be protected, more so than a standard magazine. That first round needs to be secured as well, and dirt/moisture kept out.
I think there is a good reason why FN has the cup on their pouches.
There are plenty of good PS90 pouches out there, but no great ones. Maybe the Vest Guy can make the first great one....